This invention is directed to a heat absorbing, neutral gray-green colored glass composition. More particularly, it comprises soda-lime-silica glass which uses titanium dioxide in combination with other coloring components which consist essentially of iron oxide, selenium and cobalt oxide to provide a particular excitation purity and color to a glass composition.
The majority of glass used in automotive vehicles, e.g., for the windshield and backlite, is green in color and has heat absorbing properties. A glass of green color is used because it coordinates well with a wide range of vehicle paint colors. It is desirable to have still other green or neutral gray-green glasses available which are similar in color to such glass, yet in addition have lower infra red transmittance and lower total solar energy transmittance. This combination of properties would make these other glasses ideally suited be used as other windows of the vehicles, e.g., those back of the B-pillar or the sunroof while color coordinating with the windshield glass. Such lower transmittance glasses would aid in reducing the heat gain in the vehicle interior while also providing privacy. Glass with these properties would be highly desirable for not only automotive applications but for architectural use as well. It would further be desirable if the glass exhibited a broad range of visible transmittance and was also compatible with flat glass manufacturing methods.
Those skilled in the art know that adding or substituting one colorant for another and/or changing the amount or relative proportion of colorants in a glass composition affects not only the color of the glass, i.e., dominant wavelength and excitation purity, but also can affect the light transmission of the glass and its structural qualities. Furthermore, there is in many cases substantial complexity and unpredictability in these effects. Thus, for example, even if the proper alteration in the composition of a particular colored glass were determined for achieving a desired color shift, the same alteration, unfortunately, would also alter (for example, unacceptably reduce) the light transmission value of the glass. It may, of course, also undesirably alter the purity of the glass color. In short, all these factors--dominant wavelength, purity and light transmission--are variable and may vary unpredictably with each other. In the case of heat absorbing glass, additional properties of ultraviolet transmittance and infra red transmittance are also to be considered.
Developing a new glass composition, therefore, having a particular color, certain low purity, appropriate light transmittance, ultraviolet transmittance and infra red transmittance, is in some cases very difficult. An experimental change in the amount or relative proportions of one or more colorants in a glass composition intended to bring one of these numerical values closer to a target value causes one or all of the other values simultaneously to drift off target (or further off target). I have found that obtaining my desired glass composition required maintaining the coloring components of iron oxide, selenium, cobalt oxide and titanium oxide within precise ranges.
Gray colored heat absorbing glasses relied, in the past, on the inclusion of nickel oxide as an active coloring agent. Nickel compounds, however, are known to react with other materials in soda-lime-silica glass and form nickel sulfide "stones" in the glass. These stones are usually small, thereby avoiding detection methods, but can produce an unacceptably high rate of breakage during tempering. U.S. Pat. No. 5,023,210 to Krumwiede et. al. discloses the use of chrome oxide in combination with iron oxide, cobalt oxide and selenium to achieve a dark gray glass without nickel. U.S. Pat. No. 3,300,323 to Plumat, et al. discloses a gray glass having coloring components: 0.05 to 0.5 wt. % Fe.sub.2 O.sub.3, 2.0 to 6.0 wt. % TiO.sub.2, 0.005 to 0.010 CoO, and optionally manganese oxide. The Plumat, et al. glass does not contain selenium, however, as required in the present invention and has higher visible transmittance and limited heat absorbing properties.
Other heat absorbing gray glass compositions contain selenium as an essential coloring component. For example, U.S. Pat. No. 4,873,206 to Jones discloses a gray glass composition which includes as the colorants only iron oxide, cobalt, and selenium. This composition, however, generally uses less iron oxide than this invention and has substantially higher infra-red and ultraviolet transmittance which limits the value as an automotive product. European Patent 0 482 535 to Longobardo, et al., while using iron oxide, cobalt oxide, and selenium in forming a gray glass, specifically excludes titanium oxide which is considered a vital part of the present invention glass composition to ensure its neutral gray to green color.